1. Field of the Invention
The present invention generally relates to a tilt-type electrical switch and, more particularly, is concerned with a mercury tilt switch having a unique construction and method of manufacture.
2. Description of the Prior Art
Mercury tilt switches are used in a wide variety of applications, such as in thermostats, float controls, solenoids, relays, etc. All of these applications involve initiating an electrical switching action by mechanical movement or tilting of the switch. Specifically, the making or breaking of electrical contact occurs as a result of tilting of the switch which causes a quantity of mercury contained therein to flow from one location to another.
The contact electrodes of the mercury tilt switch are typically a pair of spaced electrodes disposed at one end of a hollow housing of the switch. When the switch housing is composed of electrically conductive material, one of the electrodes can be the switch housing itself and the other an electrode in an assembly received in the one end of the hollow housing fixed in an insulated relationship thereto. The switch is closed and electrical contact made when the switch housing is tilted in such a manner that the quantity of mercury flows toward and collects at the one end of the switch housing where the mercury bridges the spaced electrodes. On the other hand, the switch is opened and electrical contact broken when the switch housing is tilted in such opposite manner that the quantity of mercury flows toward and collects at the opposite end of the switch housing out of contact with at least one of the electrodes.
Representative of the prior art are the mercury tilt switches disclosed in U.S. Pat. Nos. to Staley (2,182,216), Posey et al., (2,545,629), Bucklen III., et al., (2,570,095), Ray et al., (2,799,753), Mallatratt (3,474,203), Camin (3,983,350), Becker (4,434,337) and Johnston (4,529,854 and 4,572,934).
One widely used prior art mercury tilt switch construction, being similar to that shown in the Johnston patents, is illustrated in FIGS. 1 and 2 herein. Basically, the prior art mercury tilt switch 10 of FIG. 1 includes a deep drawn steel cylindrical housing 12 having opposite closed and open ends 14 and 16 respectively. An electrode assembly, generally indicated as 18, is coaxially inserted into and affixed to the open end 16 of the housing 12 so as to close the housing 12 and form a gas tight seal. The electrode assembly 18 includes a one-piece solid electrode 20, an annular glass insulating member 22 sealed about the electrode 20 and a steel weld ring 24 fitted about the glass insulating member 22 and adapted to be affixed, such as by resistance welding, to the open end 16 of the housing 12.
The mercury tilt switch 10 of FIG. 1 has a relatively narrow angle of operation as compared to the switch 10' of FIG. 2. As is well-known, the addition of an apertured baffle member 26 to the switch 10' of FIG. 2 permits switch 10' to have a relatively wide angle of operation in that tilting of the switch 10' can cover a much wider angle than that of switch 10 before sufficient flow of the mercury either into or out of contact with the electrodes will occur. The baffle member 26 is typically electrically conductive and coaxially mounted inside of the housing 12 by an interference fit therewith intermediate of the closed and open ends 14 and 16 thereof. While the housing 12 alone of the switch 10 of FIG. 1 constitutes the other electrode, both the housing 12 and baffle member 26 of the switch 10' of FIG. 2 constitute the other electrode. Baffle member 26 may be non-conductive such that housing 12 alone may serve as the other electrode.
One serious drawback in regard to either the prior art narrow or wide angle switches 10 and 10' relates to the type of glass-to-metal seal employed by both. The seal is ordinarily what is termed a "matched" seal wherein the glass and metal are selected so as to have substantially the same coefficients of expansion over a predetermined range of temperatures. This temperature range, for instance 50 to 600 degrees F., includes the minimum and maximum temperatures normally expected in the operating environment of the switch as well as the maximum temperature expected to be reached during the welding of the electrode assembly 18 in the open end 16 of the switch. However, the matched seal is very fragile and vulnerable to cracking in response to either mechanical shock or heat shock of the welding operation. Thus, it is difficult to produce a high percentage of switches of this prior art construction which have leak-free seals.
What is commonly known as a compression seal has also been utilized with thin-walled housings such as housing 12 of FIGS. 1 and 2. However, to accommodate the compression seal, it has been necessary to provide a heavier annular ring or grommet to contain the high stresses in the glass. This generally increases the size of the mercury switch, as well as the cost. Accordingly, although a generally more rugged compression-type seal may be utilized, the cost thereof, due to the increased cost of a heavy annular ring, makes this type of switch less desirable.
Another drawback in regard to either of the prior art narrow or wide angle switches 10 and 10' relates to the manufacture of "short run" switches or, as more often referred to, the manufacture of a small number of a particular switch generally for very specific uses or for testing purposes. For these short runs, it becomes quite impractical and much too costly to manufacture the deep drawn steel cylindrical housing necessary for switches 10 and 10'. Accordingly, a need exists for a mercury switch and method of construction whereby short run mercury switches can be manufactured in a less costly manner than having to prepare the necessary drawing dies and deep drawing steel cylindrical housings such as those of switches 10 and 10'.
Another mercury tilt switch construction, which differs from the switch construction of FIGS. 1 and 2 herein, is disclosed in the above-cited patent to Ray et al. Referring to the single figure of the Ray et al., patent and the reference numerals used therein, the switch disclosed therein has a hollow steel casing 1 which is open at both ends. One end of the casing 1 is hermetically sealed to a glass insulator 2 through which electrical conductors 5 and 6 extend into the casing 1. A metal end cap 7 is positioned within and is hermetically sealed to the other end of the casing 1. Interposed in spaced relation between the end cap 7 and the insulator 2 is another metal cap 8 which is secured, for example, by pressure fitting, to the inner wall of the casing 1. A mercury pool 9 is contained in the inner chamber of the casing between the insulator 2 and the inner cap 8. The end cap 7 is assembled to the other open end of the casing 1, for example by welding, after the inner cap 8 has been placed in position within the casing 1. Thus, no mercury can come in contact with the end cap 7 during the welding operation and, thus, there is no opportunity for the end cap 7 when heated to contact and react with the mercury which later can cause erratic mercury flow and unreliable switch performance.
However, the switch construction of Ray et al., appears to create more problems than it solves. By requiring the placement of a pressure-fitted inner cap 8 within the casing 1 to keep the mercury away from the outer end cap 7 as the latter is heated for sealing, Ray et al., disadvantageously employ additional steps and need extra components to manufacture their switch and, thus, also substantially increase the cost of the switch. Also, the added length of the casing required to provide the outer chamber between the end cap 7 and inner cap 8 disadvantageously increases the overall size of the switch and cost of the casing material. Further, although the patent to Ray et al., is silent on the exact type of seal provided between the glass insulator 2 and the casing 1, it is believed that at the time period of this patent, it would have been of the above-described matched seal type. Thus, the seal at the interface of the casing 1 and insulator 2 in Ray et al., probably suffers from substantially the same drawbacks as discussed above with respect to the prior art mercury tilt switches of FIGS. 1 and 2 herein.
Consequently, in view of the above-described drawbacks and problems, it is readily apparent that a need still exists for a fresh approach to mercury tilt switch construction which will eliminate such drawbacks and problems without creating new ones in their place.